In addition to the system catalogs, PostgreSQL provides a number of built-in
views. Some system views provide convenient access to some
commonly used queries on the system catalogs. Other views provide
access to internal server state.

The information schema (Chapter 30) provides an alternative
set of views which overlap the functionality of the system views.
Since the information schema is SQL-standard whereas the views
described here are PostgreSQL-specific, it's usually better to
use the information schema if it provides all the information you
need.

Table 41-31 lists
the system views described here. More detailed documentation of
each view follows below. There are some additional views that
provide access to the results of the statistics collector; they
are described in Table
23-1.